More frequent buses tends to have a split effect. The first-order effect is linearly fewer passengers per bus. The second-order effect of people now switching to buses has to be quite strong (and often is) to overcome this direct effect.

I happened to travel from Amsterdam to Boston today (missing King’s Day). There was no chance I wouldn’t take the train to the airport on the Amsterdam end. There was equally no chance that I would take mass transit on the Boston end, despite living in Cambridge.